Fuel cells, such as proton exchange membrane fuel cells (abbreviated as PEMFC hereafter) are a promising alternative for supplying clean electric energy to stationary or mobile applications. However in spite of their excellent performances, their economical development is hampered by the high cost of their constitutive materials and the insufficient durability of the materials making up the membrane-electrodes assembly during real operation.
Within the scope of the present invention, by clean operation of a fuel cell is meant variations of potential caused by changes in the power output from the cell. At full power, the potential of the cathode may fall to 0.5V relatively to the relative hydrogen electrode (RHE), or even to 0 V/RHE in the case of flooding/total oxygen consumption, while it may rise to 1 V/RHE in an open circuit (zero current) or more (up to 1.5 V/RHE) in transient starting/stopping phases and local fuel depletion at the anode.
The performance of the catalytic layer of a PEMFC is governed by several processes which have to be optimized simultaneously:                ion conduction ensured by the ionomer,        electron conduction ensured by the carbonaceous support and        diffusion of the reagent and of the reaction products in its porosity.        
In order to increase the factor of use ucatalyst (the fraction of the catalyst which is both in ion percolation with the membrane (via the ionomer of the catalytic layers) or in electron percolation (via the carbonaceous support)), metal deposition vacuum techniques (sputtering, plasma vapor deposition, chemical vapor deposition) allowing deposition of controlled amounts of catalyst (preferably platinum) at the surface of a gas diffusion electrode have demonstrated an interesting potential.
The electrocatalysts used in the catalytic layers of PEMFC may be:                solid platinum nanoparticles which are supported on carbon with a high specific surface area. This type of electrocatalyst is abbreviated hereafter as: Pt/C;        solid multi-metal nanoparticles, i.e. in addition to platinum they contain other metal elements (generally transition metals) and are supported on a carbon with high specific surface area. This type of electrocatalyst is abbreviated hereafter as: PtxM/C.        
Within the scope of the present invention, the nanoparticules are supported on a carbon, which means that the nanoparticles are generally, because of physisorption phenomena, laid on the carbon and interact with the carbon support through dipolar or weak bonds.
The carbon support has the function of stabilizing the nanoparticles which, in its absence, would agglomerate with each other. This would lead to:                a dispersion loss (the ratio between the number Nsurface of platinum atoms present at the surface and the total number Ntotal of atoms contained in a nanoparticle which is practically expressed as the ratio of electrochemically active surface area/platinum mass used),        small electrochemically weak active areas, and        poor distribution of the nanoparticles in the volume of the electrode.        
These consequences would not be compatible with good electrical performances.
The size of solid electrocatalyst nanoparticles is generally comprised between 1 and 5 nm.
At the anode of the PEMFC, the platinum electrocatalyst on a carbon support (Pt/C) is used for oxidizing dihydrogen into protons and electrons.
At the cathode of the PEMFC, the platinum electrocatalyst on a carbon support (Pt/C) catalyzes the oxygen reduction reaction leading to the formation of water. This reaction occurs at a high potential, or the order of 0.6 to 1 V/HRE. The result of this is instability of the platinum nanoparticles and therefore a loss of electrochemically active surface area of the nanoparticles, because of aging of the Ostwald maturation type.
Indeed, the platinum nanoparticles may be corroded into Ptz+ ions (with z=2.4). This corrosion preferentially occurs on the smallest platinum nanoparticles, as shown by Gibbs-Thompson equation (Equation 1) below, wherein μi is a chemical potential, νm is the volume of an atom and γ the surface tension:
                              μ                      i            ,                          (              d              )                                      =                              μ                          i              ,                              (                                  d                  =                  8                                )                                              +                                    4              ⁢                              υ                m                            ⁢              γ                        d                                              Equation        ⁢                                  ⁢        1            
This equation shows that due to the excess surface energy, the chemical potential of small size nanoparticles is higher than that of a massive material μi,(d=∞).
The produced Ptz+ ions (z=2,4) may be re-deposited electrochemically (reduction by the electrochemical potential) or chemically (reduction by the dihydrogen from the anode) on larger size nanoparticles (for which the standard potential is higher) resulting in fine in an increase in the average size of the electrocatalyst nanoparticles. The aforementioned equation 1 shows that the latter will then be more stable towards this phenomenon, since the more electrocatalyst nanoparticles are of a significant size, the more they are stable.
By decreasing the size of the platinum nanoparticles for their use as an electrocatalyst in PEMFCs, it is possible to increase the dispersion of the platinum. In terms of mass activity of the electrocatalyst, the optimum is nevertheless obtained with diameters of the order of 3 to 4 nm, a size which is difficult to control for electrocatalysts for which the platinum mass percentage referred to the carbon substrate mass (i.e. mpt/(mpt+mc)) is greater than 30%, because of the marked trend to agglomeration of the platinum nanoparticles, this reduces their dispersion by as much.
However, the decrease in the size of the platinum nanoparticles is not fully satisfactory, because the kinetics of the oxygen reduction reaction decreases consequently, when the size of the nanoparticles is less than the size of the order of 3 to 4 nm.
This is why, as mentioned above, in the technical field of PEMFC electrolyzers, platinum alloyed to a second less noble element such as a transition metal (PtxM/C avec M=Co, Ni, Cu, Cr for example and x comprised between 0.2 and 10, preferably between 0.5 and 3) is used.
This has the two following advantages:
(i) decrease in the mass of precious metal (i.e. platinum) used in the catalytic layer and therefore in its cost,
(ii) multiplication of the specific activity for the oxygen reduction reaction (the ratio of the catalytic activity to the area of the catalyst) by a factor from 2 to 4 as compared with a Pt/C type electrocatalyst as detailed above.
As the ageing mechanisms are similar from a morphological point of view on electrocatalysts of the Pt/C type or of the PtxM/C type, Ostwald maturation also remains a problem during the operation of a PEMFC cathode. Ageing is even worsened by the presence of a less noble metal M (and therefore more corrodible) than the platinum. The metals M alloyed to platinum are actually unstable from a thermodynamical point of view. Severe depletion of the less noble metal is seen, not only at the surface but also in the core of the material of the electrocatalyst during operation of the PEMFC. This causes marked decrease in the specific activity (down to values sometimes less than that of platinum) and severe poisoning of the ionomer and of the proton exchange membrane by the thereby produced My+ ions. Of course, the larger size of the bimetal crystallites gives the possibility of attenuating the significance of this phenomenon.
The Ptz+ (z=2, 4) and My+ ions (in the case of multi-element alloys for an electrocatalyst of the PtxM/C type) produced by the corrosion of the nanoparticles are redistributed in the membrane-electrode assembly by the two following phenomena:                migration (electric potential gradient), and        diffusion (chemical potential gradient).        
When the PEMFC operates, migration theoretically maintains these ions in the cathode catalytic layer, which causes poisoning of the sulfated terminations of the ionomer (ion conductor) and a strong decrease in the electrocatalytic activity for the oxygen reduction reaction of the electrocatalyst of the type PtxM/C in contact with this ionomer. These ions may diffuse into the proton exchange membrane and into the gas diffusion layer upon interrupting the operation of the PEMFC (for example during a maintenance operation).
Further, in addition to the electrocatalyst nanoparticles being subject to electrochemical corrosion combined with ageing of the Ostwald maturation type during the operation of a PEMFC, the following physical phenomena occur and also contribute to causing a time-dependent change in the structure and the chemical composition of the electrocatalyst:                1) The carbonaceous support is strongly oxidized during certain stress modes of the PEMFC, such as the stopping and starting periods of the latter.        2) The electrocatalyst crystallites are not motionless at the surface of the carbonaceous support but are coalescent after surface diffusion (migration of the crystallites). This phenomenon may be accelerated by the corrosion of the carbonaceous support.        